Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates to hydraulic systems for operating machinery, and in particular to electronic control systems for operating electrohydraulic valves to control the flow of fluid to and from hydraulic actuators.
2. Description of the Related Art
A wide variety of machines have moveable members which are operated by an hydraulic actuator, such as a cylinder and piston arrangement or hydraulic motor, that is driven by the flow of fluid controlled by a hydraulic valve. Traditionally the hydraulic valve was manually operated by the machine operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid operated valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near an operator station, but can be located adjacent the actuator being controlled. This change in technology also facilitates computerized control of the machine functions.
Proportional solenoid operated spool valves are well known for controlling the flow of hydraulic fluid. That type of valve employs an electromagnetic coil which moves an armature connected to the spool, the position of which determines the amount of fluid flow through the valve. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or the spool is spring loaded to close the valve when electric current is removed from the solenoid coil. Alternatively a second electromagnetic coil and armature is provided to move the spool in the opposite direction.
When an operator desires to move a member on the machine, a joystick is operated to produce an electrical signal indicative of the direction and desired rate at which the corresponding hydraulic actuator is to move. The faster the actuator is desired to operate, the farther the joystick is moved from its neutral position. A control circuit receives a joystick signal and responds by producing an electric current of a given magnitude which opens the associated valve to achieve the proper movement of the actuator.
The control of an entire machine, such as an agricultural tractor or construction apparatus is complicated by the need to control multiple functions simultaneously. For example, control of a backhoe often requires simultaneous operation of the separate hydraulic actuators for the boom, arm, bucket, and swing. In some cases, the aggregate amount of hydraulic fluid flow being demanded by the simultaneously operating functions exceeds the maximum flow that the pump is capable of producing. At such times, it is desirable that the control system allocate the available hydraulic fluid among those functions in an equitable manner, so that one function does not consume a disproportionate amount of the available hydraulic fluid flow.
A typical hydraulic system has a supply line that carries pressurized fluid from a source such as a pump, a return line which carries fluid back to a tank, and at least one hydraulic actuator coupled by a separate valve assembly to the supply line and the return line. A control system operates the valve assemblies in response to an operator input to move each hydraulic actuator as desired by the operator.
The control system includes a user input device operable by the machine user to generate an input signal indicating desired movement of the actuator. A mapping routine converts the input signal into a velocity command designating a desired velocity for the actuator. That velocity command indicates the direction and rate of motion. A valve opening routine converts the velocity command into a set of valve flow coefficients for the valve assembly and, from the set of valve flow coefficients, a set of control signals is produced which designates levels of electric current to apply to valves within the valve assembly. A plurality of valve drivers applies electric current to valves within the valve assembly in response to the set of control signals.
A pressure controller also may be provided to regulate pressure in the supply line in response to the velocity command, thereby ensuring that a suitable pressure is available to power the actuator.
In the preferred embodiment of the invention, a selector is provided to choose a metering mode in which the hydraulic function is to operate. For example, the metering mode is selected in response to the velocity command and force acting on the actuator.
When the hydraulic system has a plurality of functions, a flow sharing routine in included to allocate fluid flow from the supply line equitably to each of the plurality of functions. For example, the flow sharing routine varies the velocity command for each function when the aggregate flow being demanded by the plurality of functions exceeds the total flow available from the supply line.